1. Field of the Invention
The present invention relates generally to photoconductor structures including photosensitive elements responsive to incident radiation and more particularly to photoconductor structures having a wide range of response including sensitivity in the blue and red light wavelength regions.
2. Description of the Prior Art
It has been recognized in the prior art that photoconductor elements exhibit lower sensitivity at short or blue wavelengths than at higher or red wavelengths. One way to provide uniform sensitivity over both ranges of wavelength is to employ a filter or the like to reduce the sensitivity of the device at the red color to conform to the lower sensitivity at the blue color. This of course results in a photoconductor having an overall low sensitivity. An example of a structure which is designed to provide increased sensitivity in the blue light range rather than employ a filter is described in U.S. Pat. No. 3,900,882 issued Aug. 19, 1975 to Fukai et al on an application entitled "Photoconductor Element", filed Mar. 29, 1974 and assigned to Matsushita Electric Industrial Co. In this patent a photoconductor element is composed of selected materials such as An, Cd, Te and In arranged in a hetero-junction in order to provide the aforesaid increased blue light sensitivity. The present invention is quite distinct from the Fukai et al patent and other known prior art in that it does not employ selected materials to form a complex hetero-junction but instead uses an ion implant and a phosphorous diffusion to create a built in electric field to prevent carrier surface recombination.
It is also recognized in the prior art that photodetectors for laser detection can be fabricated such that the doping concentration of the layers is selected to create a high electric field distribution in the multiplying junction and a lower electric field in the absorber so that the lower field in the absorber moves the photogenerated carriers toward the multiplying junction. An example of such device is described in U.S. Pat. No. 3,889,284 issued June 10, 1975 to Schiel on an application entitled "Avalanche Photodiode with Varying Bandgap", filed Jan. 15, 1974 and assigned to the United States of America.
The present invention is distinct from the Schiel type prior art in that such art does not relate to improving the lower blue wavelength response of a photodiode by the type of shallow implant to be described. Specifically, the Schiel patent uses added InAs doping to lower the device bandgap as the wavelength of the light being detected increases.
Finally attention is directed to U.S. Pat. No. 3,873,828 issued Mar. 25, 1975 to Hunsperger et al on an application entitled "Integrated Optical Detector" filed Aug. 23, 1973 and assigned to Hughes Aircraft Company. This patent discusses the use of ion implantation in a semiconductor optical detector, however the implantation is employed to create active defect centers to shift an absorption edge to render the substrate less absorbing. This use of ion implantation is totally distinct from the principles of the present invention.